Yeast Sir2 is a nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase that plays a central role in transcriptional silencing, chromosomal stability, DNA damage response and aging. In mammals, Sir2-like genes constitute a seven-member family whose function is largely unknown. To investigate the role of the Sir2 family in vertebrates, we have disrupted Sir2 homologues SIRT1 and SIRT2 in the p53-deficient chicken cell line DT40. Both SIRT1(-/-) and SIRT2(-/-) cells had mild growth defects. Colony survival assays showed moderate and mild sensitivity to cisplatin in SIRT1(-/-) and SIRT2(-/-) cells, respectively, while SIRT1(-/-), but not SIRT2(-/-) cells, were sensitive to ionizing radiation (IR). Cells rendered doubly deficient in SIRT1 and SIRT2 exhibited the same levels of IR and cisplatin sensitivity as SIRT1(-/-) cells. SIRT1(-/-) cells appeared to be defective neither in DNA double strand break repair nor in G2/M checkpoints, but were more susceptible to cell death induction following IR than wild-type cells. Furthermore, both SIRT1- and SIRT2-deficient cells were more sensitive to pro-apoptotic stimuli including cisplatin and staurosporine. Our results indicate that SIRT1 and SIRT2 regulate stress-induced cell death pathways in a p53-independent manner.